1. Field of the Invention
The present invention relates to a broadband wireless access communication system, and more particularly to a system and method for controlling the operation mode of a Medium Access Control (MAC) layer.
2. Description of the Related Art
In 4th generation (4G) communication systems, which are the next generation communication systems, improvements focus on providing various qualities of service (QoSs) at high transmission speed. The third generation (3G) communication system supports a transmission speed of about 384 kbps outdoors with relatively bad channel conditions and a maximum transmission speed of about 2 Mbps indoors with relatively good channel conditions
Wireless Local Area Network (LAN) and Metropolitan Area Network (MAN) communication systems generally support transmission speeds of 20 to 50 Mbps. Since the wireless MAN communication system has a wide service coverage and supports a high transmission speed, it is suitable for supporting a high speed communication service. However, the wireless MAN system does not provide for mobility of a user, i.e., a subscriber station (SS), or a handover for high speed movement of the SS.
As a result, in 4G communication systems, a new type of communication system ensuring mobility and QoS for wireless LAN and MAN systems supporting relatively high transmission speeds is being developed to support high speed service in the 4G communication system.
The IEEE (Institute of Electrical and Electronics Engineers) 802.16a communication system employs an Orthogonal Frequency Division Multiplexing (OFDM) scheme and an Orthogonal Frequency Division Multiple Access (OFDMA) scheme to support a broadband transmission network for a physical channel of the wireless MAN system.
The IEEE 802.16a communication system considers only a single cell structure and stationary SSs, so the system does not consider movement of the SSs. In contrast, an IEEE 802.16e communication system has been defined as a system designed for mobility of an SS in addition to the IEEE 802.16a communication system, and thus, should reflect mobility of an SS in a multi-cell environment. To provide for the mobility of an SS in a multi-cell environment as described above, of the operation mode changes of the SS and its base station (BS) are considered and accommodated. To that end, research about SS handover in a multi-cell structure is actively pursued to support SS mobility. Herein, a mobile SS is referred to as an Mobile Subscriber Station (MSS).
FIG. 1 is a block diagram schematically illustrating the structure of a IEEE 802.16e communication system.
Referring to FIG. 1, the IEEE 802.16e communication system has a multi-cell structure with a cell 100 and a cell 150. In addition, the IEEE 802.16e communication system includes a BS 110 controlling the cell 100, a BS 140 controlling the cell 150, and a plurality of MSSs 111, 113, 130, 151, and 153. The transmission/reception of signals between the BSs 110 and 140 and the MSSs 111, 113, 130, 151, and 153 is accomplished through an OFDM/OFDMA scheme. The MSS 130 is located in a boundary zone (i.e., handover zone) between the cell 100 and the cell 150. When a handover for the MSS 130 is possible, the MSS 130 can move without loss of service.
In the IEEE 802.16e communication system, a certain MSS receives pilot signals transmitted from a plurality of BSs and measures Carrier to Interference and Noise Ratios (CINRs) of the received pilot signals. The MSS selects the BS with the highest CINR as a serving BS, which means the MSS belongs to that BS. The MSS, having selected the serving BS, receives the downlink frame and uplink frame transmitted from the serving BS and uses them in transmitting and receiving data.
In the case where mobility of the MSS is taken into consideration as described above, MSS power consumption plays an important part in system performance. Therefore, a sleep mode operation and an awake mode operation have been proposed for the BS and the MSS to minimize MSS power consumption.
Hereinafter, operation modes of a Medium Access Control (MAC) layer for the IEEE 802.16e communication system will be described with reference to FIG. 2.
FIG. 2 is a mode diagram schematically illustrating the operation modes supported by a MAC layer of the IEEE 802.16e communication system.
Referring to FIG. 2, the MAC layer of the IEEE 802.16e communication system supports two kinds of operation modes (i.e., an awake mode 210 and a sleep mode 220). First, the sleep mode 220 has been proposed in order to minimize the power consumption of the MSS during idle time when packet data is not transmitted. The MSS mode-transits (211) from the awake mode 210 to the sleep mode 220, thereby minimizing the power consumption of the MSS during the idle time when packet data is not transmitted. In general, packet data is transmitted in bursts when generated. It would be inefficient to perform the same operations when data is transmitted and when data is not transmitted. For this reason, the sleep mode operation as described above has been developed.
When packet data is generated while the MSS is in the sleep mode, the MSS mode-transits into the awake mode and transmits/receives the packet data. However, because the packet data is highly reliable on a traffic mode, the sleep mode operation must be organically performed in consideration of the traffic characteristic and the transmission scheme characteristic of the packet data.
Hereinafter, schemes proposed up to now for the IEEE 802.16e communication system to support operation in the sleep mode 220 will be described.
First, to mode-transit into the sleep mode 220, an MSS receives mode transition consent from a BS. The BS allows the MSS to shift into the sleep mode 220 simultaneously while buffering or dropping the packet data to be transmitted to the MSS. In addition, the BS informs the MSS of packet data to be transmitted during the listening interval of the MSS. The MSS awakes from the sleep mode 220 and checks whether there is any packet data to be transmitted from the BS to the MSS. The listening interval will be described below in more detail. When there is packet data to be transmitted from the BS to the MSS, the MSS mode-transits to the awake mode 210 from the sleep mode 220 and receives the packet data from the BS. When there is no packet data to be transmitted from the BS to the MSS, the MSS stays in the sleep mode 220.
Hereinafter, parameters to support operation in the sleep mode and the awake mode will be described.
1) Sleep Interval
The sleep interval is an interval requested by an MSS and assigned by a BS according to the MSS request. The sleep interval also represents the time it takes to go from the sleep mode 220 to the awake mode 210. In other words, the sleep interval is defined as an interval during which the MSS stays in the sleep mode 220. The MSS may continue to stay in the sleep mode 220 even after the sleep interval is over. In this case, the MSS updates the sleep interval by performing a sleep interval update algorithm by means of a preset initial sleep window value and a final sleep window value. Herein, the initial sleep window value corresponds to a minimum sleep window value and the final sleep window value corresponds to a maximum sleep window value. Further, both the initial sleep window value and the final sleep window value are assigned by the BS and expressed by the number of frames. Since the minimum window value and the maximum window value will be described in detail below, a further description is omitted here.
2) Listening Interval
The listening interval is an interval requested by an MSS and assigned by a BS according to the MSS request. Further, the listening interval represents the time it takes for the MSS to awake from the sleep mode 220 and synchronize with the downlink signal of the BS sufficient enough to decode downlink messages such as a traffic indication (TRF_IND) message. Herein, the TRF_IND message is a message representing existence of traffic (i.e., packet data) to be transmitted to the MSS. Since the TRF_IND message will be described below, a further detailed description is omitted here. The MSS determines whether to stay in the awake mode or to mode-transit back into the sleep mode according to the values of the TRF_IND message.
3) Sleep Interval Update Algorithm
When the MSS goes into the sleep mode 220, it determines the sleep interval while regarding the preset minimum window value as a minimum sleep mode interval. After the sleep interval passes, the MSS awakes from the sleep mode 220 for the listening interval and checks whether there is packet data to be transmitted from the BS. If there is no packet data to be transmitted, the MSS renews the sleep interval to be twice as long as that of the previous sleep interval and continues to stay in the sleep mode 220. For example, when the minimum window value is “2”, the MSS sets the sleep interval to be 2 frames and stays in the sleep mode for 2 frames. After passage of the 2 frames, the MSS awakes from the sleep mode and determines whether the TRF_IND message has been received. When the TRF_IND message has not been received (that is, when no packet data transmitted from the BS to the MSS exists), the MSS sets the sleep interval to be 4 frames (twice as many as 2 frames) and stays in the sleep mode 220 during the 4 frames. In this way, the sleep interval increases within a range from the initial sleep window value to the final sleep window value. The algorithm for updating the sleep interval as described above is the sleep interval update algorithm.
Hereinafter, a network re-entry process of an MSS will be described with reference to FIG. 3.
FIG. 3 is a signal flowchart schematically illustrating a network re-entry process of an MSS in a conventional IEEE 802.16e communication system.
First, in step 311, according to handover, the MSS receives preambles of downlink frames transmitted from the handovered BS (i.e. a new serving BS) and acquires system sync with the new serving BS. Thereafter, the MSS acquires downlink sync from BS information contained in messages broadcasted by the BS, which include a Downlink Channel Descriptor (DCD) message, an Uplink Channel Descriptor (UCD) message, a downlink map (DL_MAP) message, an uplink map (UL_MAP) message, a mobile neighbor advertisement (MOB_NBR_ADV) message.
Thereafter, in step 313, the MSS transmits a ranging request (RNG_REQ) message to the BS, receives a ranging response (RNG_RSP) message from the BS in response to the RNG_REQ message, and acquires uplink sync with the BS from the RNG_RSP message. Then, in step 315, the MSS adjusts frequency and power.
Thereafter, in step 317, the MSS negotiates the basic capacity of the MSS with the BS. In step 319, the MSS acquires an Authorization Key (AK) and a Traffic Encryption Key (TEK) by performing authentication operation together with the BS. In step 321, the MSS requests the BS to register the MSS and the BS completes registration of the MSS. In step 323, the MSS performs an Internet Protocol (IP) connection with the BS. In step 325, the MSS downloads operational information through the IP in connection with the BS. In step 327, the MSS performs service flow connection with the BS. Here, the service flow refers to a flow in which MAC_SDUs (service data units) are transmitted and received through a connection having a certain, predetermined threshold QoS. Thereafter, in step 329, the MSS uses the service provided from the BS. Then, the process ends.
Next, a handover process in an IEEE 802.16e communication system will be described with reference to FIG. 4.
FIG. 4 is a signal flow diagram schematically illustrating a handover process in a conventional IEEE 802.16e communication system.
Referring to FIG. 4, the MSS scans CINRs of the pilot signals from the neighbor BSs in the process described (step 411). When the MSS 400 determines that it should change the serving BS (step 413), the MSS 400 transmits an Mobile Handover Request (MOB_HO_REQ) message to the current serving BS 410 (step 415). FIG. 4 is based on an assumption that the MSS 400 has two neighbor BSs including a first BS 420 and a second BS 430. Here, the MOB_HO_REQ message includes the result of scanning by the MSS 400.
When the serving BS 410 receives the MOB_HO_REQ message, the serving BS 410 detects information on a list of neighbor BSs to which the MSS 400 can be handed over from information contained in the received MOB_HO_REQ message (step 417). Here, for the convenience of description, the list of neighbor BSs to which the MSS 400 can be handed over will be referred to as ‘handover-available neighbor BS list’, and this example assumes that the handover-available neighbor BS list includes the first BS 420 and the second BS 430. The serving BS 410 transmits a handover notification (HO_NOTIFICATION) message to the neighbor BSs contained in the handover-available neighbor BS list, i.e., the first BS 420 and the second BS 430 (steps 419 and 421).
Upon receiving the HO_NOTIFICATION message from the serving BS 410, each of the first BS 420 and the second BS 430 transmits a handover notification response (HO_NOTIFICATION_RESPONSE) message, which is a response message to the HO_NOTIFICATION message, to the serving BS 410 (step 423 and 425). The HO_NOTIFICATION_RESPONSE message contains a plurality of Information Elements (IEs) including an MSS ID of the MSS 400, a response (ACK/NACK) regarding whether or not the neighbor BSs can perform the handover in response to the request of the MSS 400, and bandwidth and service level information which each of the neighbor BSs can provide when the MSS 400 is handed over for each BS.
When the serving BS 410 has receives the HO_NOTIFI-CATION_RESPONSE message transmitted from the first neighbor BS 420 and the second neighbor BS 430, the serving BS 410 selects a neighbor BS that can optimally provide a bandwidth and a service level requested by the MSS 400 when the MSS 400 is handed over, as a target BS to which the MSS 400 will be actually handed over.
For instance, if the service level required by the MSS 400 is higher than a service level which can be provided by the first neighbor BS 420 and is equal to a service level which can be provided by the second neighbor BS 430, the serving BS 410 will select the second neighbor BS 430 as the target BS. Then, the serving BS 410 transmits a handover notification confirmation (HO_NOTIFICATION_CONFIRM) message to the second neighbor BS 430 as a response to the HO_NOTIFICATION_RESPONSE message (step 427).
The serving BS 410 transmits an Mobile handover response (MOB_HO_RSP) message to the MSS 400 as a response to the MOB_HO_REQ message (step 429). The MOB_HO_RSP message contains information on the target BS to which the MSS 400 will be handed over.
Next, upon receiving the MOB_HO_RSP message, the MSS 400 analyzes the information contained in the MOB_HO_RSP message and selects the target BS. After selecting the target BS, the MSS 400 transmits an Mobile handover indication (MOB_HO_IND) message to the serving BS 410 as a response to the MOB_HO_RSP message (step 431).
Upon receiving the MOB_HO_IND message, the serving BS 410 recognizes that the MSS 400 will be handed over to the target BS (i.e., the second neighbor BS 430) contained in the MOB_HO_IND message, and then releases the present setup link with the MSS 400 (step 433). Then, the MSS 400 performs an initial ranging process with the second neighbor BS 430 (step 435) and performs a network re-entry process with the second neighbor BS 430 when the initial ranging succeeds (step 437).
The handover-related operations as described with reference to FIG. 4 are operations performed by the MSS in awake mode. However, when the MSS detects that it has reached a cell boundary zone while in sleep mode, the MSS switches to the awake mode and performs the handover-related operations of FIG. 4. In other words, when the MSS moves from a first cell to a second cell in sleep mode, the MSS cannot restore the connection with the first cell BS and performs a network re-entry process with the second cell BS. In performing the network re-entry process in the current IEEE 802.16e communication system, the MSS transmits an BS identifier (BS ID) of the previous BS to which the MSS belonged, so that the new BS can recognize that the MSS is being handed over. Then, the new BS can acquire information of the MSS from the previous BS and perform the handover together with the MSS.
The above description is given on both a method for reducing power consumption of an MSS and a method for handover of an MSS. However, when the method for reducing power consumption is applied to an MSS in the sleep mode, the method becomes inefficient because the MSS, although it is in sleep mode, must perform the handover as described above whenever it shifts between cells, especially when even an MSS having no traffic to transmit or receive at all must perform the handover whenever it shifts between cells. The effect MSS power consumption reduction is lessened and message overhead is generated during the handover operation. Furthermore, all MSSs in the sleep and awake modes perform periodic ranging. This, too, causes unnecessary power consumption and generates message overhead.
Further, the current IEEE 802.16e communication system constantly assigns various types of basic radio resources to MSSs with no traffic to transmit or receive. Below are the basic radio resources that are always assigned regardless of actual need.
(1) Basic Connection Identifier (CID) (Basic CID)
The basic CID is a connection identifier used in transmitting a message that is relatively short and must be urgently transmitted (i.e., an urgent control message).
(2) Primary Management CID
The primary management CID is a connection identifier used in transmitting a message that is relatively long and has a relatively lower urgency.
(3) Secondary Management CID
The secondary management CID is a connection identifier used in transmitting a message that has a relatively lower urgency and relates to a standard protocol for at least three layers.
Furthermore, in the IEEE 802.16e communication system, each MSS is assigned an Internet Protocol version 4 (IPv4) address which is also a limited radio resource. As described above, in the IEEE 802.16e communication system, radio resources as described above, such as the connection identifiers and IPv4 addresses, may be assigned to MSSs having no data to transmit or receive, thereby degrading the efficiency in use of radio resources. Therefore, there is a necessity for a specific operation scheme of a MAC layer to support operation between a BS and an MSS, that can maximize efficiency in use of radio resources while minimizing power consumption of the MSS moving at high speeds.